Q-switched lasers are often based on Neodymium-doped Yttrium Aluminum Garnet (Nd:YAG) as the laser material. This material lases at several wavelengths including 1064 nanometers. Such lasers often have application not at the 1064-nm wavelength of Nd:YAG, but at the harmonics of Nd:YAG: 532 nm and 355 nm, and perhaps in the future at higher harmonics. Frequency converting crystals, e.g. Lithium Triborate (LBO) convert the 1064-nm fundamental radiation from the Nd:YAG to the higher harmonics. The efficiency of conversion to the harmonics is quite good at low (20 kHz and under) repetition rates, with the power at the harmonic being 50% or more of the power that can be achieved at the fundamental infra-red wavelength. However, in many laser applications, such as precision micro-machining, higher repetition rates, e.g., as high as 100 kHz and above are desired. Such high repetition rates allow removal of material in small increments, in a way that is cleaner and more precise than is possible at low repetition rates.
Unfortunately, at these high repetition rates, the efficiency of conversion to the harmonics is greatly reduced. A laser which can be specified at an average power of 10 Watts of 355 nm output at a repetition rate of 10 kHz can only provide 2 Watts at 200 kHz. It would be desirable to maintain the high efficiency over a much higher range of repetition rates.
In addition, at high repetition rates, the Q-switched pulses lengthen as well as decline in energy, so that the peak power of the laser is greatly reduced. For some applications, the peak power goes below the threshold at which materials processing can take place. It would be desirable to maintain a high peak power, even as the energy per pulse is reduced at high repetition rate.
In the unsaturated regime, efficiency of non-linear frequency conversion is proportional to peak power, and high peak power can result in higher conversion efficiency. The peak power can be enhanced by optical amplification. Unfortunately, amplification of a broadened pulse to high peak powers can damage non-linear materials used for frequency conversion.
Thus, there is a need in the art, for a laser system that overcomes the above disadvantages.